The present invention relates to a pointing mechanism for use in an electric propulsion system of a spacecraft and an electric propulsion system for use in a spacecraft which is equipped with a pointing mechanism of this kind.
In general, electric propulsion systems are used for spacecraft propulsion which enable movement and maneuvering of a spacecraft equipped therewith. Specifically, electric propulsion systems are usually equipped with at least one electric thruster, such as an ion engine, which, upon accelerating ions or plasma, generates thrust acting on the spacecraft. In order to optimize electric thruster propellant resources, to limit a number of electric thrusters used within an electric propulsion system, and to enable advanced attitude and orbit maneuvering, such systems are equipped with a pointing mechanism, so-called electric propulsion pointing mechanism (EPPM), configured to direct a thrust vector generated by the at least one electric thruster of the electric propulsion system in a desired direction relative to a center of gravity of the spacecraft. The pointing mechanism comprises a mobile plate for receiving a thruster which is supported movably relative to a spacecraft structure. Specifically, for accelerating a spacecraft, as opposed to a maneuvering movement of the spacecraft, currently deployed pointing mechanisms are adapted to perfectly align the thus generated thrust vector such that it passes through a center of gravity of the spacecraft. As a result, to allow for a multi-axis alignment of the thrust vector of the electric propulsion system, the pointing mechanism is typically provided with at least two degrees of freedom.
For example, pointing mechanisms in the form of a so-called Gough-Stewart platform are known, in which the mobile plate is supported by six actuators mounted to a spacecraft structure which are attached in pairs to three positions on the mobile plate. In this way, the mobile plate is provided with six degrees of freedom, i.e., the mobile plate can be moved translationally along and rotably around the spacecraft's principal axes, namely the roll, pitch and yaw axes.
Further, pointing mechanisms are known, in which the mobile plate is mounted on a pair of gimbals to allow rotating the thrust vector generated by a thruster about both the pitch and the yaw axes of the spacecraft.
AT 507 213 A4 2010-03-15 discloses a pointing mechanism comprising a mobile plate which is driven by at least two rotary actuators connected in series, the rotational axes of which are disposed in parallel and displaced to one another.
The center of gravity of a spacecraft, e.g., due to propellant consumption, usually changes during the operation of the spacecraft. To that end, also the thrusters itself are subjected to a misalignment, e.g., due to forces acting thereon during a start of the spacecraft, abrasion, etc. As a result, such systems require a plurality of highly precise sensors and actuators for aligning the at least one thruster, thereby imposing computing effort on a spacecraft's control system, i.e., an attitude and orbit control system (AOCS).